Name: ______________________________
Block: ______ Date: __________________
Percent Abundance Practice
1. Argon has three naturally occurring isotopes: argon-36, argon-38, and argon-40. Based on
argon’s reported atomic mass (look on the periodic table), which isotope do you think is the
most abundant in nature? Explain.
2. Copper is made of two isotopes. Copper-63 is 69.17% abundant and it has a mass of 62.9296
amu. Copper-65 is 30.83% abundant and it has a mass of 64.9278 amu. What is the weighted
average mass of these two isotopes?
3. Calculate the atomic mass of silicon. The three silicon isotopes have atomic masses and relative
abundances of 27.9769 amu (92.2297%), 28.9765 amu (4.6832%) and 29.9738 amu (3.0872%).
4. Gallium has two naturally occurring isotopes. The mass of gallium-69 is 68.9256 amu and it is
60.108% abundant. The mass of gallium-71 is 70.9247 amu and it is 39.892% abundant. Find
the atomic mass of gallium.
5. Bromine has two naturally occurring isotopes. Bromine-79 has a mass of 78.918 amu and is
50.69% abundant. Using the atomic mass reported on the periodic table, determine the mass
of bromine-81, the other isotope of bromine.
6. Calculate the atomic mass of lead. The four lead isotopes have atomic masses and relative
abundances of 203.973 amu (1.4%), 205.974 amu (24.1%), 206.976 amu (22.1%) and
207.977 amu (52.4%).
7. Naturally occurring europium (Eu) consists of two isotopes was a mass of 151 and 153.
Europium-151 has an abundance of 48.03% and Europium-153 has an abundance of
51.97%. What is the atomic mass of europium?
13
8. Here are three isotopes of an element: 126C
6C
a. The element is:
b. The number 6 refers to the
c. The numbers 12, 13, and 14 refer to the
d. How many protons and neutrons are in the first isotope?
e. How many protons and neutrons are in the second isotope?
f. How many protons and neutrons are in the third isotope?
14
6C
9. Complete the following chart:
Isotope name
atomic #
mass #
# of protons
# of neutrons
# of electrons
92 uranium-235
92 uranium-238
5 boron-10
5 boron-11
10. Antimony has two naturally occurring isotopes. The mass of antimony-121 is 120.904 amu
and the mass of antimony-123 is 122.904 amu. Using the average mass from the periodic
table, find the abundance of each isotope. (Remember that the sum of the two abundances
must be 100).
Yes, I’m aware this one is hard, but give it a try, you might surprise yourself. 
11.